Add update target to ninja

[Black]

Summary

I just added an update command to ninja, now we can do $ ninja update, as we can do $ make update.

[chandlerc]

I'm strongly opposed to wiring the build system and version control system together.

CMakeLists.txt

   endif()
 endif()
 
+if(CMAKE_GENERATOR MATCHES "Ninja")
+  add_custom_target(update
+      COMMAND find "${CMAKE_CURRENT_SOURCE_DIR}" -name .svn -type d | sed -e 's/\\.svn//' | xargs svn update
+  )
+endif()